GPU not recognized on 2.8
The requirements for GPU rendering are higher than the minimum requirements for running Blender.


  Supported Graphics Cards
  
  These requirements are for basic Blender operation, rendering using the GPU requires slightly different requirements.


The manual states that GCN second generation or later is required for GPU rendering. Graphics cards that do not meet that requirement will not be listed in the preferences.


  AMD OpenCL
  
  OpenCL is supported for GPU rendering with AMD graphics cards. Blender Supports graphics cards with GCN generation 2 and above. To make sure your GPU is supported, see the list of GCN generations with the GCN generation and supported graphics cards.
  
  AMD OpenCL GPU rendering is supported on Windows and Linux, but not on macOS.


Since the Radeon R9 280 has the GCN first generation architecture it's not supported for GPU rendering.